1. Field of the Invention
The present invention relates to a Si-base composition in the form of a slurry comprising water as the dispersion medium, which is used to produce silicon nitride (Si3N4) ceramics, as well as the high strength Si3N4 ceramics produced from the Si-base composition in the form of a slurry, and a method for producing them.
2. Description of the Related Art
To produce Si3N4 ceramics having good electrical, thermal, and mechanical properties, it is essential to prepare a precursor (slurry composition) comprising the uniform dispersion and mixture of a fine, high purity starting material powder and other additives.
Alcohols have been used as dispersion media to improve the dispersion properties of the Si powder and to control oxidation at the surface of the Si powder when Si3N4 ceramics are produced from slurry compositions comprising silicon (Si) powder starting material. However, processes featuring the use of alcohols suffer from problems in terms of the cost of the product, such as the need to use expensive explosion-proof facilities.
The ability to use water as the dispersion medium, on the other hand, would eliminate the need for explosion-proof facilities, and would allow Si3N4 ceramics to be produced at a lower cost. Examples of existing means for using water as a dispersion medium have been given in Japanese Laid-Open Patent Applications (Kokai) 3-261662 and 1-212279, Japanese Patent 2686248, and U.S. Pat. No. 5,387,562.
Among these slurry compositions comprising water as a dispersion medium and the methods for producing Si3N4 ceramics using such compositions, no sintering aid necessary for the compaction of the ceramic is added to the slurry in the methods given in Japanese Laid-Open Patent Applications (Kokai) 3-261662 and 1-212279. The resulting Si3N4 sintered body thus contains an abundance of small gas holes dispersed throughout. As a result, the Si3N4 sintered body has an extremely low flexural strength of no more than 600 MPa. No sintering aid is added to the slurry in these methods because the dispersion properties between the Si powder and sintering aid are not very good in water, resulting in a decline in the inherently outstanding properties of Si3N4.
U.S. Pat. No. 5,387,562 discloses means for adjusting the pH to the alkaline end of the scale in order to improve the dispersion properties between the Si powder and sintering aid in water. That is, sintering aids Y2O3 and Al2O3 are added to Si powder pre-milled in an organic solvent or to Si powder preheated in a 200 to 300xc2x0 C. gas, and the ingredients are mixed in alkaline water with a pH of 11 to 13. Alternatively, a Si powder containing the same sintering aids is heated in air in the same manner as above, and the ingredients are then similarly mixed in alkaline water. The water content of the slurries in these methods is no more than 50 wt %.
However, in the method disclosed in the U.S. patent, the Si powder is milled in an organic solvent such as an alcohol, making it necessary to subsequently oxidize the surface in air. When the Si powder is milled all at once in the water, the surface of the fine, active particles gradually forms a SiO2 film as a result of dehydration, while the reaction is accelerated as the mean particle size of the Si powder becomes smaller. Therefore, with reduction in Si particle size, Si particles thus tend to aggregate in the slurry during or after the milling process for producing the fine Si particles, and thus prevents a slurry with good dispersion properties from being obtained, while also causing the resulting sintered body to suffer deterioration of properties, poor coloration, and the like. Since the water content of the slurry is no more than 50 wt %, as noted above, the composition sometimes becomes non-homogeneous when prepared.
Japanese Patent 2686248 discloses a method in which 50 to 90 wt % water and as much as 7.5 wt % of a sintering aid, as calculated in terms of oxides, are added to a Si powder preheated to between 200 and 800xc2x0 C. in the atmosphere, and a small amount of a dispersion agent such as an ammonium polycarboxylate is also added to adjust the pH of the slurry to an alkalinity of between 8 and 12. However, since the Si particles are similarly dispersed in water in this method as well, the surface must be pre-oxidized.
In conventional methods for preparing slurries for the manufacture of Si3N4 ceramics in this manner, it is impossible to simultaneously control the oxidation of the Si powder and improve the dispersion properties of the fine Si powder in aqueous solvent, as long as water is used as the dispersion agent. Not only is it difficult to handle the fine Si powder, but it is also necessary to employ at least some economically disadvantageous means such as oxidation of the Si powder in air.
In view of the foregoing, an object of the present invention is to improve the oxidation resistance and dispersion properties of Si powder in water so as to provide a Si-base composition in the form of a slurry suitable for the manufacture of Si3N4 ceramics, as well as an inexpensive manufacturing method. Another object of the present invention is to provide Si3N4 ceramics with better electrical, thermal, and mechanical properties by using such a slurry Si-base composition.
To achieve the aforementioned objectives, the Si-base composition in the form of a slurry proposed in the present invention comprises a Si powder with a mean particle size of no more than 10 xcexcm and a surface oxygen content of no more than 3 wt %, a sintering aid, water in an amount of 50 to 90 wt % relative to the total weight of the composition, and a surface coating agent in an amount of 0.05 to 10 wt % relative to the Si powder, wherein the pH is adjusted to between 3 and 8.
The surface coating agent in the aforementioned Si-base composition in the form of a slurry is trivalent metal ions of at least one selected from the group consisting of Fe, Ga, Sc, Al, Cr, Yb, Sm, Y, In, Pb, V, Ti, Zr, and Hf, or a Si polymer or polymeric compound with an HLB of no more than 10.
HLB is an abbreviation for hydrophile-lipophile balance, which represents the balance between the hydrophilic groups and lipophilic groups (hydrophobic groups) in the surface coating agent. The number of hydrophilic groups and number of lipophilic groups are determined based on the molecular structure, allowing the HLB to be calculated based on the following equation 1.
HLB=7+xcexa3 (number of hydrophilic groups)xe2x88x92xcexa3 (number of lipophilic groups)xe2x80x83xe2x80x83[Equation 1]